A process of this kind is disclosed in EP-A-378,279, in which a solution of polyethene in decalin is extruded to form a film from which the decalin is removed by evaporation after cooling to below the gelling point of the solution, during which process the solution is converted into a gel. During said evaporation, the shrinkage occurring in the film is prevented. The resulting film from which the solvent has been removed already has an initial porosity of about 30-50% and subsequently even becomes highly porous, with a porosity of greater than 90%, as a result of biaxial stretching.
This known film has the disadvantage that, although the tensile strength has an adequate value, the bursting strength is low. Consequently, the film can only be used to a limited extent in membrane applications, where in many cases a difference in pressure has to be maintained across the film.